The compression of a digital image reduces the quantity of data necessary for its transmission via, for example, a telecommunication network such as the Internet network.
Certain compression standards, such as the MPEG-4 standard (Motion Picture Expert Group 4), use a so-called multilayer compression according to which an image, the so-called source image, is compressed by means of a base layer, coding a so-called base image, and of an enhancement layer, coding the deviation or the residual between the source image and the base image.
In this case, the transmission of a compressed image can be done by transmitting, firstly, its base layer and then, secondly, its enhancement layer, in the form of binary trains.
The binary train associated with the enhancement layer has the advantage of being able to be truncated in such a way that, if transmission thereof is performed only partially following, for example, a variation in the transmission throughput in the network, the information transmitted in the correctly received part of the binary train may nevertheless still be used to enhance the image coded by the base layer.
To do this, a residual 101 (FIG. 1), coding enhancement coefficients 1011, 1012, . . . 101n specific to each block of pixels forming the image, is decomposed into various planes MSB, MSB−1, MSB−2, MSB−3 and LSB of bits ranked in order of priority, each plane MSB, MSB−1, MSB−2, MSB−3 or LSB clustering together bits of like weight in the decomposition of the residual specific to each block.
As described later, these coefficients 1011, 1012, . . . 101n may arise from the discrete cosine transformation DCT of an 8×*8 block of pixels, performed by evaluating the difference between the DCT coefficients of a block of a source image, that is to say of the coded image, and the DCT coefficients of an image processed by a quantization and then by an inverse quantization of its base layer.
By generating a binary train 130 successively transmitting the planes MSB, MSB−1, MSB−2, MSB−3 or LSB, one then obtains an enhancement layer which can be truncated without compromising the transmission of the train as a whole. For example, if the transmission of the train 130 is disturbed during the transmission of the plane MSB−3, the information as a whole already transmitted by the planes MSB, MSB−1, MSB−2 and the transmitted part of MSB−3 will be able to be taken into account to enhance the image.
Such a method, dubbed Fine Granularity Scalability (Fgs) within the framework of the MPEG4 standard, has the drawback of using a compression of the residual having an efficiency lower than the efficiency of the compression of the base code. In this way, a compromise between the efficiency of the compression, increased by a base layer of significant size, and the security of the transmission of the base, increased with a base layer of small size, may possibly be made.